A color material having a property in which its color is decolorized by applying heat to cut coupling between a pigment and a color forming agent is conventionally known. Use of such a color material as a developing agent in an image forming apparatus employing electrophotographic process enables reuse of printing sheets of paper. This reduces energy required for manufacture and recycling of sheets of paper, thereby contributing to reduction of carbon dioxide emissions.
If used in an image forming apparatus employing electrophotographic process, a decolorable developing agent using the aforementioned decolorable color material (such a developing agent is hereinafter called a decolorable toner) forms a developed image on an image supporting member such as a photosensitive drum. The developed image formed of the decolorable toner is transferred to a printing sheet of paper at a transfer position. The printing sheet of paper holding the unfixed decolorable toner image is conveyed to a fixing unit. Then, the fixing unit applies heat of predetermined fixing temperature and pressure to the unfixed decolorable toner image, thereby fixing the unfixed decolorable toner image on the printing sheet of paper. Fixing of the unfixed decolorable toner image is started at temperature lower than that of heat to be applied to start decolorization of the fixed decolorable toner image.
Regarding a non-decolorable toner image, fixing of the non-decolorable toner image may be started at relatively high temperatures. Accordingly, even if temperature varies during continuous printing and duplex printing, or even (low) ambient temperature changes, fixing performance can be maintained at a satisfactory level.
Meanwhile, regarding an unfixed decolorable toner image, fixing of the unfixed decolorable toner image is started at temperature that generally does not differ largely from temperature at which decolorization is started. Thus, temperature control of the fixing unit in consideration of temperature at which decolorization is started has been required.